Photovoltaic (PV) devices can provide a portable source of electrical power for a wide variety of commercial and defense applications. Many of these mobile and portable power applications can directly benefit from the development of flexible, lightweight, and high-efficiency photovoltaic sheets. Emerging technical approaches for achieving flexible photovoltaic power include the growth of copper indium gallium diselenide (CIGS) cells on flexible substrates and the epitaxial liftoff (ELO) of III-V devices onto thin metal film handles. With both approaches, however, advanced designs are needed; designs that can further increase power generating performance, lower module weight, enhance robustness, and reduce costs.
III-V photovoltaic devices employing nano-inspired technologies can dramatically outperform CIGS cells, enabling the design of flexible PV sheets that combine ultra-low weight with ultra-high power density. Nano-enhanced solar cells seek to harness a wide spectrum of photons at high voltages in a single junction device by embedding narrow energy-gap wells within a wide energy-gap matrix. By avoiding the limitations of current matching inherent in multi junction devices, nano-enhanced broadband solar cells have the potential to deliver ultra-high efficiency over a wide range of operating conditions. Since their initial suggestion by researchers at Imperial College, quantum well solar cells have been demonstrated in a variety of different material systems, and the basic concept has been extended to include quantum dots. Clear improvements in infrared spectral response have been experimentally confirmed in both quantum well and quantum dot solar cells. However, photon absorption, and thus current generation, is hindered in conventional nano-enhanced solar cells by the limited path length of incident light passing vertically through the device. Moreover, the insertion of narrow energy-gap material into the device structure often results in lower voltage operation, and hence in lower PV power conversion efficiency. Both of these issues are addressed with the novel device structure disclosed herein.